As a technique for achieving a high density magnetic recording, a perpendicular magnetic recording has been targeted as an alternative to a conventional longitudinal magnetic recording. In a magnetic recording layer of a perpendicular magnetic recording medium, a crystalline film of CoCr alloy having a hexagonal close packed (hcp) crystal structure principally has been studied. To make a perpendicular magnetic recording possible, the crystal alignment is controlled so that the c-axis of each crystal grain is perpendicular to the film surface, i.e., the c-plane parallel to the film surface. For addressing a higher recording density in a perpendicular magnetic recording medium, attempts have been made to minimize the grain size and to reduce the dispersion of the grain size of the CoCr alloy composing the magnetic recording layer, and to reduce magnetic interaction between crystal grains.
A technique has been proposed to achieve a high density recording in a longitudinal magnetic recording medium by reducing magnetic interaction between crystal grains, in which a layer of nonmagnetic and non-metallic substance such as oxide or nitride, is formed at the grain boundary of a crystal grain composing a magnetic recording layer. This magnetic layer is called “a granular magnetic layer,” and disclosed in Japanese Unexamined Patent Application Publication Hei 8-255342 and U.S. Pat. No. 5,679,473, for example. The magnetic grains in the granular magnetic layer are three dimensionally isolated with each other by the nonmagnetic and non-metallic grain boundary. Since magnetic interaction between magnetic grains decreases, “zigzag domain wall” in the transition region of a recording bit is suppressed, which is considered to result in noise reduction.
On this background, the use of the granular magnetic layer has been proposed in the magnetic recording layer of a perpendicular magnetic recording medium. For example, IEEE Trans. Mag., vol. 36, p. 2393 (2000) discloses a perpendicular magnetic recording medium that comprises an underlayer of ruthenium (Ru) and a magnetic layer of a CoPtCrO alloy having a granular structure. The document shows that the c-axis alignment of crystal grains composing the magnetic layer enhances as the ruthenium underlayer becomes thicker, bringing about improvement in magnetic characteristics and electromagnetic conversion performances of the magnetic recording layer. However, the thickness of the ruthenium (Ru) underlayer of at least 40 nm is necessary to obtain a granular perpendicular magnetic recording medium that exhibits excellent characteristics.
A so-called double-layered perpendicular magnetic recording medium has been proposed, where a soft magnetic backing layer is provided under the magnetic recording layer to enhance sharpness of the magnetic field produced at the head position. To achieve a high density recording in a magnetic recording medium with this structure, it is considered essential that the thickness of the nonmagnetic layer provided between the magnetic recording layer and the soft magnetic layer be 20 nm or thinner. This places a restriction on the magnetic recording medium. Even in a structure without the soft magnetic backing layer, it is desirable for the thickness of the ruthenium underlayer to be thin from the viewpoint of reducing the manufacturing cost since ruthenium is an expensive rare metallic element.
Accordingly, there is a need for a perpendicular magnetic recording medium that exhibits excellent magnetic characteristics and superior electromagnetic conversion performance at a low cost. The present invention addresses this need.